Brazil’s prison authorities have discovered that they have been holding a man – who was arrested in 1960 – for 23 years after he was supposed to have been released.

The unnamed man, who has not been named and is aged 80, should have been freed in 1989 but has been locked up in the psychiatric unit of the Governor Stenio Gomes prison in Itaitinga, Ceara, north-east Brazil.

He was discovered wearing diapers and sitting in a wheelchair during an inspection in August by a team of investigators from the Conselho Nacional de Justica (CNJ – National Council of Justice).

“I think this man is the oldest prisoner in Brazil since the information that is entered in the prison records show that he was arrested in the Sixties in the last century,” Judge Paulo Augusto Irion said.

According to Mr Irion, the man was arrested in Ceara for a crime that has not been disclosed. He was given parole in 1989 but never left.

The judge is part of a team of government investigators carrying out intensive inspections of the prison system in 14 states. The aim is to identify illegal arrests, sentencing irregularities, ensure compliance with the rights of detainees and review prison conditions and standards. An explanation as to why the prisoner has languished behind bars for so long was given as possibly family abandonment and the prison authorities failing to find suitable accommodation to release him into.

This is the third time that Ceara has received a visit from the inspectors. The first was in 2009 and the second in 2011.

On the last visit, the inspectors examined 6,500 cases and about 1,200 prisoners were released following evidence of irregularities. According to data from the Department of Justice, there are around 19,665 prisoners in Ceara whose prison terms need to be urgently reviewed.

“In this institute this year, I came across six persons whose sentences had expired, but who were still being kept in jail,” said Mr Irion, who gave no further explanation as to why this man’s case has only now just come to light.

According to the International Centre for Prison Studies, Brazil has a prison population of 548,003 the fourth highest in the world.

A Human Rights Watch report in 2012 found that detention centres in Brazil “are severely overcrowded, lengthy pre-trial detention is the norm and torture continues to be a serious problem. Forced labour persists in some states despite federal efforts to eradicate it”.

Almost one-sixth of Brazil’s prison population could do time outside prison, critics say.